The present invention relates to a driving circuit for a solenoid pump or the like, in which a driving current supplied from a rechargeable battery is alternately switched to on and off when flowing it in an inductive element for driving the solenoid pump, thereby generating a driving force.
Conventional solenoid pump driving circuits of the type to which the present invention relates will be described with reference to FIGS. 1 through 3.
According to the circuit shown in FIG. 1 which comprises an inductive element 10, a switching element 11 and a battery 12, when the switching element 11 is on-off controlled, a counter-electromotive force is generated in the inductive element 10. Therefore, the switching element 11 must withstand a voltage as great as the source voltage plus the induced reverse voltage.
The Japanese Utility Model Laid-Open Publication No. 62-15743 discloses a driving circuit as shown in FIG. 2 in which the circuit is designed to comply with the above requirement. In this circuit, a first rechargeable battery 36 serving to supply a source voltage as well as to clamp a voltage induced by a counter-electromotive force, has positive terminal coupled to the fixed contact of a first changeover switch 35. The first changeover switch 35 also has two movable contacts A1 and B1. The first battery 36 is coupled through the contact B1 to one terminal of an inductive element 30 serving as a plunger driver. Another terminal of the inductive element 30 is coupled to the negative terminal of the first battery 36 through a switching element 32. A second rechargeable battery 34 also serving to supply a source voltage and clamp a voltage induced by a counter-electromotive force, has a negative terminal coupled to the negative terminal of the first battery 36 and a positive terminal coupled to the fixed contact of a second changeover switch 33. The second changeover switch 33 also has movable contacts A2 and B2. The contacts A1 and B2 are coupled to each other. A reverse-current blocking diode 37 is interposed between a first node connecting the inductive element 30 and the switching element 32 and second node connecting the contacts A1 and B2, in which the anode of the diode 37 is connected to the first node and the cathode thereof to the second node. The changeover switches 33 and 35 are ganged together so as to be simultaneously changed over. Specifically, those switches 33 and 35 are changed respectively over to contacts A2 and A1 or to contacts B2 and B1.
If, in the above circuit, the switching element 32 is closed, and at the same time, the changeover switches 33 and 35 are changed over to their respective contacts B2 and B1, a current flows in a loop defined by the first battery 36, changeover switch 35, inductive element 30 and changeover switch 32, to thus drive a plunger.
When the switching element 32 is then opened, a current is generated due to the counter-electromotive force of the inductive element 30. And, this current flows through a loop defined by the inductive element 30, diode 37, contact B2, changeover switch 33, rechargeable batteries 34 and 36, and changeover switch 35 in the stated order, wherein a current is regenerated from the battery 34.
When the switching element 32 is closed at the same time when the changeover switches 33 and 35 are thrown to their respective terminals A2 and A1, a current flows through a loop defined by the second battery 34, changeover switch 33, contact A2, inductive element 30 end switching element 32 in the stated order. As a result, the plunger is again driven.
When the switching element 32 is subsequently opened, a current induced again due to the counter-electromotive force of the inductive element 30 flows through a loop defined by the inductive element 30, the diode 37, contact A1, batteries 36 and 34, changeover switch 33 and contact A2, wherein a current is regenerated from the first battery 36. This arrangement increases the durations of the rechargeable batteries 34 and 36.
Japanese Patent Application Laid-Open Publication No. 59-65581 discloses another driving circuit as shown in FIG. 3 which is also designed to comply with the aforementioned requirement regarding the withstand voltage. In this circuit, a rechargeable battery 20 is coupled to both a first series circuit comprising a first diode 21 and a first transistor 22 and a second series circuit comprising a second transistor 32 and a second diode 24. The first and second diodes are reverse biased with respect to the DC power source 20. A solenoid coil or an inductive element 25 for driving a plunger is coupled between the first and second series circuits and an input device 26 is coupled to the bases of the transistors 22 and 23 so as to simultaneously turn or turn off the same. When the transistors 22 and 23 are simultaneously turned off, a current induced due to the counter-electromotive force of the solenoid coil 25 flows through the first diode 21, rechargeable battery 20 and second diode 24, wherein a current is regenerated from the battery 20.
These two types of conventional driving circuits have the following shortcomings. The first driving circuit shown in FIG. 2 requires two rechargeable batteries 34 and 36 for supplying source voltages and clamping a voltage induced by a counter-electromotive force, as well as a relay circuit for changing over two switches 33 and 35.. This increases the manufacturing cost, the number of causes for possible damages and the weight of the driving circuit. Further, an undue counter-electromotive force induced by the opening of the switching element 32 causes to flow a current greater than the rated charge currents of the rechargeable batteries 34 and 36 through these batteries. This deteriorates the batteries 34 and 36, which may result in liquid leakage and reduction in their abilities.
With regard to the second driving circuit, an undue counter-electromotive force induced by the turning off of the transistors 22 and 23 causes to flow a current greater than the rated charge currents of the rechargeable battery 20 therethrough. This deteriorates the battery 20, and thus results in liquid leakage and reduction in the ability of the battery 20.